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Title Environmental impacts of utility-scale solar energy

Permalink https://escholarship.org/uc/item/62w112cg

Journal Renewable and Sustainable Energy Reviews, 29

ISSN 1364-0321

Authors Hernandez, RR Easter, SB Murphy-Mariscal, ML et al.

Publication Date 2014

DOI 10.1016/j.rser.2013.08.041

Peer reviewed

eScholarship.org Powered by the California Digital Library University of California Renewable and Sustainable Energy Reviews 29 (2014) 766–779

Contents lists available at ScienceDirect

Renewable and Sustainable Energy Reviews

journal homepage: www.elsevier.com/locate/rser

Environmental impacts of utility-scale solar energy

R.R. Hernandez a,b,n, S.B. Easter b,c, M.L. Murphy-Mariscal d, F.T. Maestre e, M. Tavassoli b, E.B. Allen d,f, C.W. Barrows d, J. Belnap g, R. Ochoa-Hueso h,S.Ravia, M.F. Allen d,i,j a Department of Environmental Earth System Science, Stanford University, Stanford, CA, USA b Department of Global Ecology, Carnegie Institution for Science, Stanford, CA, USA c Ecofactor, Redwood City, CA, USA d Center for Conservation Biology, University of California, Riverside, CA, USA e Departamento de Biología y Geología, Universidad Rey Juan Carlos, Móstoles, Spain f Department of Botany and Plant Science, University of California, Riverside, CA, USA g U.S. Geological Survey, Southwest Biological Science Center, Moab, UT, USA h Hawkesbury Institute for the Environment, University of Western Sydney, Penrith, 2751, New South Wales, Australia i Department of Biology, University of California, Riverside, CA, USA j Department of Plant Pathology, University of California, Riverside, CA, USA article info abstract

Article history: Renewable energy is a promising alternative to fossil fuel-based energy, but its development can require Received 22 February 2013 a complex set of environmental tradeoffs. A recent increase in solar energy systems, especially large, Received in revised form centralized installations, underscores the urgency of understanding their environmental interactions. 29 July 2013 Synthesizing literature across numerous disciplines, we review direct and indirect environmental Accepted 11 August 2013 impacts – both beneficial and adverse – of utility-scale solar energy (USSE) development, including impacts on biodiversity, land-use and land-cover change, soils, water resources, and human health. Keywords: Additionally, we review feedbacks between USSE infrastructure and land-atmosphere interactions and Biodiversity the potential for USSE systems to mitigate climate change. Several characteristics and development Conservation strategies of USSE systems have low environmental impacts relative to other energy systems, including Desert other renewables. We show opportunities to increase USSE environmental co-benefits, the permitting Greenhouse gas emissions Land use and land cover change and regulatory constraints and opportunities of USSE, and highlight future research directions to better Photovoltaic understand the nexus between USSE and the environment. Increasing the environmental compatibility Renewable energy of USSE systems will maximize the efficacy of this key renewable energy source in mitigating climatic and global environmental change. & 2013 Elsevier Ltd. All rights reserved.

Contents

1. Introduction ...... 767 2. Environmental impacts of utility-scale solar energy systems ...... 768 2.1. Biodiversity ...... 769 2.1.1. Proximate impacts on biodiversity ...... 769 2.1.2. Indirect and regional effects on biodiversity ...... 769 2.2. Water use and consumption ...... 770 2.3. Soil erosion, aeolian sediment transport, and feedbacks to energetic efficiency ...... 770 2.4. Human health and air quality ...... 770 2.5. Ecological impacts of transmission lines and corridors ...... 771 2.6. Land-use and land-cover change ...... 771 2.6.1. Land-use dynamics of energy systems ...... 771 2.6.2. Land-use of utility-scale solar energy ...... 771 2.6.3. Comparing land-use across all energy systems ...... 773 3. Utility-scale solar energy, land-atmosphere interactions, and climate change ...... 773

n Corresponding author. Department of Environmental Earth System Science, 51 Dudley Lane, Apt 125, 260 Panama Street, Stanford, CA 94305, USA. Tel.: þ1 650 681 7457. E-mail address: [email protected] (R.R. Hernandez).

1364-0321/$ - see front matter & 2013 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.rser.2013.08.041 R.R. Hernandez et al. / Renewable and Sustainable Energy Reviews 29 (2014) 766–779 767

3.1. Utility-scale solar energy and albedo ...... 773 3.2. Utility-scale solar energy and surface roughness ...... 773 3.3. Utility-scale solar energy and climate change ...... 773 4. Utility-scale solar energy co-benefit opportunities ...... 774 4.1. Utilization of degraded lands ...... 774 4.2. Co-location with agriculture ...... 775 4.3. Hybrid power systems...... 775 4.4. Floatovoltaics ...... 775 4.5. Photovoltaics in design and architecture ...... 775 5. Minimizing adverse impacts of solar energy: Permitting and regulatory implications ...... 775 6. Solar energy and the environment: Future research ...... 776 6.1. Research questions addressing environmental impacts of utility-scale solar energy systems ...... 776 6.2. Research questions addressing utility-scale solar energy, land-atmosphere interactions, and climate change...... 776 6.3. Research questions addressing utility-scale solar energy co-benefit opportunities ...... 776 6.4. Research questions addressing permitting and regulatory implications ...... 776 7. Conclusion...... 776 Acknowledgements ...... 777 References...... 777

1. Introduction positive aspects – reduction of greenhouse gases, stabilization of Renewable energy is on the rise, largely to reduce dependency degraded land, increased energy independence, job opportunities, on limited reserves of fossil fuels and to mitigate impacts of acceleration of rural electrification, and improved quality of life in climate change ([58, 110, 150]). The generation of electricity from developing countries [17,126] – that make it attractive in diverse sunlight directly (photovoltaic) and indirectly (concentrating solar regions worldwide. power) over the last decade has been growing exponentially In general, solar energy technologies fall into two broad worldwide [150]. This is not surprising as the sun can provide categories: photovoltaic (PV) and concentrating solar power more than 2500 terawatts (TW) of technically accessible energy (CSP). Photovoltaic cells convert sunlight into electric current, over large areas of Earth′s surface [82,125] and solar energy whereas CSP uses reflective surfaces to focus sunlight into a beam technologies are no longer cost prohibitive [9]. In fact, solar power to heat a working fluid in a receiver. Such mirrored surfaces technology dwarfs the potential of other renewable energy tech- include heliostat power towers (flat mirrors), parabolic troughs nologies such as wind- and biomass-derived energy by several (parabolic mirrors), and dish Stirling (bowl-shaped mirrors). The orders of magnitude [150]. Moreover, solar energy has several size and location of a solar energy installation determines whether

Fig. 1. Annual installed grid-connected photovoltaic (PV) capacity for utility-scale (420 MW) solar energy schemes and distributed solar energy schemes (i.e., non- residential and residential) in the United States. Total PV capacity was 900 MW in 2010; approximately double the capacity of 2009. Data reprinted from Sherwood [114]. Photo credits: RR Hernandez, Jeff Qvale, National Green Power. 768 R.R. Hernandez et al. / Renewable and Sustainable Energy Reviews 29 (2014) 766–779 it is distributed or utility-scale. Distributed solar energy systems environmental properties underscores the importance of under- are relatively small in capacity (e.g.,o1 megawatt [MW]). They standing environmental interactions associated with solar energy can function autonomously from the grid and are often integrated development, especially at regional and global scales and how into the built environment (e.g., on rooftops of residences, com- these impacts may reduce, augment, or interact with drivers of mercial or government buildings; solar water heating systems; global environmental change. portable battlefield and tent shield devices; [25,102]). Distributed Here, we provide a review of current literature spanning solar contrasts strikingly with utility-scale solar energy (USSE) several disciplines on the environmental impacts of USSE systems, enterprises, as the latter have relatively larger economies of scale, including impacts on biodiversity, water use and consumption, high capacity (typically 41 MW), and are geographically centralized soils, human health, and land-use and land-cover change, and —sometimes at great distances from where the energy will be land-atmosphere interactions, including the potential for USSE consumed and away from population centers. In the United States systems to mitigate climate change. Drawing from this review, we (US), solar energy has grown steadily over the past decade and show (1) mechanisms to integrate USSE environmental co-benefit rapidly in recent years (Fig. 1). The USSE capacity in this country opportunities, (2) permitting and regulatory issues related to quadrupled in 2010 from 2009, while both residential and nonresi- USSE, and (3) highlight key research needs to better understand dential capacity increased over 60% during that same period. Similar the nexus between USSE and the environment. increases in USSE have also been observed in Australia, China, Germany, India, Italy, and Spain [90,111,113,128,139]. As a paradigm of clean and sustainable energy for human use, reviews on the environmental impacts of solar energy date back to the 1970s [49,71]. For example, Lovins [71] provided a conceptual 2. Environmental impacts of utility-scale solar energy systems framework by which an energy scheme′s position along a gradient from soft (benign) to hard (harmful) is determined by the Environmental impacts (see Fig. 2 for complete list) of USSE energetic resiliency (or waste) and environmental conservation systems may occur at differential rates and magnitudes throughout (or disruption) for its complete conversion from source to final the lifespan (i.e., construction, operation, and decommission) of a end-use form. More recent reviews of the environmental impacts USSE power plant, which varies between 25 and 40 years. Drawing of solar energy systems have emphasized fundamental life-cycle from experiments evaluating direct and indirect impacts of USSE elements (upstream and downstream environmental impacts systems and studies evaluating processes that are comparable in associated with development; [126]) or were focused on specific likeness to USSE activities, we discuss impacts related to biodiversity, regions (e.g., Serbia; [90]) or fauna of interest (Lovich and Ennen, water use and consumption, soils and dust, human health and air 2012). The observed increase in USSE and studies elucidating their quality, transmission corridors, and land-use and land-cover change.

Fig. 2. Solar energy effectors for utility-scale solar energy technologies (ALL USSE), including concentrating solar power (USSE CSP) and photovoltaics (USSE PV), and for both utility-scale and distributed schemes (distributed and USSE). Effectors have one or more potential effects on the environment with one or more potential ecological responses. Photo credit: RR Hernandez. R.R. Hernandez et al. / Renewable and Sustainable Energy Reviews 29 (2014) 766–779 769

2.1. Biodiversity [19]. These and other ‘post-siting’ compliance measures to mini- mize biodiversity impacts (e.g., land acquisition, road fencing) are In general, distributed and USSE installations integrated into expensive, usually target a single species, and do not guarantee the existing built environment (e.g., roof-top PVs) will likely have benefits to the organisms they are designed to support [70]. The negligible direct effects that adversely impact biodiversity [25]. repatriation and translocation of organisms is complicated by Studies quantifying the direct impact of USSE on biodiversity in climate change, which requires taking into account the dynamic otherwise undisturbed habitats are few ([75,107]; Lovich and character of species’ distributions for both assessing biodiversity Ennen [70]; Cameron et al. [142]; [81]); however, these combined impacts of single and collective USSE projects and for determining with other disturbance-related studies provide insight into how suitable habitat for repatriation or translocation. Additionally, USSE power plants may impact biodiversity losses locally within some species, such as birds, cannot be moved and may be the USSE footprint (i.e., all areas directly transformed or impacted attracted to certain USSE infrastructural elements. McCrary [75] by an installation during its life cycle), where the aboveground found mortality rates, compared to other anthropogenic impacts vegetation is cleared and soils typically graded, and regionally by on birds, low for USSE systems, and Hernandez (unpublished data) landscape fragmentation that create barriers to the movement of observed nests on the backside of PV module infrastructure species and their genes [101]. (Fig. 3). Soil disturbances and roads can further increase mortality rates of organisms or serve as conduits for exotic invasions, which can competitively extirpate native species [42,140]. 2.1.1. Proximate impacts on biodiversity As USSE sites typically remove vegetation and soils are graded, locating USSE on land where biodiversity impacts are relatively small has been shown to be a feasible strategy for meeting both 2.1.2. Indirect and regional effects on biodiversity renewable energy and conservation goals ([39]; Cameron et al., Less proximate impacts on biodiversity may also occur indir- 2012). For example, Fluri [39] showed that the strategic siting of ectly within the USSE footprint (i.e., all areas directly transformed USSE infrastructure in South Africa could create a nominal capacity or impacted by an installation during its life cycle), beyond the of 548 gigawatts (GW) of CSP while avoiding all habitats support- footprint, and regionally by landscape fragmentation that create ing endangered or vulnerable vegetation. After a site has been barriers to the movement of species and their genes [101]. In the chosen, solar energy projects may employ repatriation and trans- southwest US, anthropogenic sources of oxidized and reduced location programs—when individuals of key native species are nitrogen may be elevated due to emissions from increased vehicle collected from impacted habitat, moved, and released into reserve activity or the use of CSP auxiliary natural gas burners, promoting areas previously inhabited and not previously inhabited by the invasions by exotic annual grasses that increase fire frequencies species, respectively. The low success rates of repatriation and [5,94]. Additionally, environmental toxicants required for USSE translocation programs (e.g.,o20%; [29,38]) have rendered them operation (e.g., dust suppressants, rust inhibitors, antifreeze an expedient when all other mitigation options are unavailable agents) and herbicides may have insalubrious, and potentially

Fig. 3. ((a) and (b)) McCrary et al. [76] documented the death of 70 birds (26 species) over 40 weeks, including effects of scavenger bias, resulting from the operation of a 10 MW concentrating solar thermal power plant (Solar One, Mojave Desert, CA; 1). This equates to a mortality rate of 1.9–2.2 individual birds per week. Two causes of death were identified: most prevalent was collision with site infrastructure (81%), particularly with heliostats, and to a lesser degree, burning when heliostats were oriented towards standby points (19%), especially for aerial foraging species. Additionally, they found that the large, man-made evaporation pools increased the number of species five-fold in the local area. Impacts on bird mortality may increase non-linearly with increasing USSE capacity. (c) Hernandez (unpublished data) observed several bird nests on the backside of PV module infrastructure at a USSE power plant in the Central Valley of California (San Joaquin Irrigation District PV Plant, Valley Home, CA, USA). Photo credit: Madison Hoffacker. 770 R.R. Hernandez et al. / Renewable and Sustainable Energy Reviews 29 (2014) 766–779 long-term, consequences on both local and regional biodiversity vegetation, but the installation of USSE infrastructure requires [1,70]. extensive landscape modification. Such modifications include Habitat loss and fragmentation are recognized as the leading vegetation removal, land grading, soil compaction, and the con- threats to biological diversity [35,136]. The land-use efficiency, struction of access roads; activities that increase soil loss by wind footprint, and infrastructural design of individual USSE installa- and water [14,37]. tions vary significantly [51] and therefore individual power plants The major agents of natural degradation are soil particulates (silt affect landscapes in unique ways. Utility-scale solar energy infra- and clay), as well other particulate pollutants such as industrial structure may fragment habitat and serve as linear barriers to the carbon (C) [98,99]. Given its variable composition, dust emissions movement patterns of certain wildlife species. Whereas highly have a broad spectrum of impacts ranging from human health, global mobile or wide-ranging species may be able to circumvent USSE biogeochemical cycle, hydrologic cycle, climate, and desertification infrastructure, some features may be insurmountable to less mobile (e.g., [46,87,88,95]). In one semiarid ecosystem, Li et al. [68] recorded species, increasing the risk of gene flow disruption between a 25% loss of total organic C and total nitrogen in the top 5 cm of soil populations. Decisions regarding the placement of USSE infrastruc- following devegetation. Studies conducted in southeast Spain have ture likely take into account current species distributions, but found that 15 years after the removal of vegetation in a semiarid site, climate change may alter future distributions and wildlife dispersal the total organic C remained 30% lower compared to undisturbed corridors [52]. Determining species’ responses to novel climate areas, which also showed greater microbial biomass and activity shifts is inherently uncertain and scale dependent, but nevertheless levels [12]. Decreases in the availability of resources resulting from tools exist to model such distributional shifts (e.g., [11]). soil erosion can result in biodiversity losses and impede the recovery of vegetation [4,47,104]. Moreover, reduction in vegetative cover are 2.2. Water use and consumption strongly linked to increased dust production and even modest reductions in grass or shrub cover have been shown to dramatically Energy and water are interdependent [129]. USSE technologies increase dust flux [68,80]. vary in their water withdrawal (total volume removed from a water Dust deposition can incur a negative feedback to solar energetic source) and consumption (volume of withdrawn water not returned performance by decreasing the amount of solar radiation absorbed to the source) rates, creating unique tradeoffs. Photovoltaic energy by PV cells [45]. Even suspended dust in the near surface atmo- systems have low rates (0.02 m3/megawatt hours [MW h]), consum- sphere decreases the amount of solar radiation reaching the panel ing water only for panel washing and dust suppression in places surface [45]. Deposition on solar panels or mirrors is site-specific where dust deposition is problematic [41]. Currently, washing panels and modulated by several factors, including soil parent material, or mirrors with water is the most common strategy for dust removal microclimate, and frequency and intensity of dust events, but in large solar installations [73]. A recent analysis of water use by several studies have demonstrated energy production losses USSE installations in the southwestern US indicates that water for exceeding 20% [33,34,45,85]. Nonetheless, long-term field studies dust control is a major component (60–99%) of total water consump- to quantify dust impacts on solar energy production are limited. tion in both dry cooled CSP and PV installations (Ravi et al., in For example, Ibrahim [55] experimentally demonstrated that solar review), whereas no information is available for other regions where modules installed in the Egyptian desert that have been exposed USSE installations are expected to increase in the near future. Even to dust for a period of one year showed an energy reduction of though other cleaning technologies (e.g., electrostatic) exist, most are about 35%. Kimber et al. [61] investigated the effects of deposition not yet commercially available, and the impacts of conventional on energy production for large grid-connected systems in the US technologies (e.g., cleaning using chemical sprays) on the environ- and developed a modeling framework for predicting soiling losses. ment are not completely understood [50,65]. These authors found that for North American deserts, PV system In the case of CSP, the water consumption depends on the efficiency declines by an average of 0.3% per day during periods cooling system adopted—wet cooling, dry cooling, or a combina- without rain [61]. The National Renewable Energy Laboratory tion of the two (hybrid cooling) [108]. Concentrating solar power analyzed 24 PV systems throughout the US and calculated a consumes vast quantities of water in wet cooling (i.e., 3.07 m3/ typical derate factor (percentage decrease in power output) due MW h), which is greater than coal and natural gas consumption to dust deposition of 0.95% [74]. In many desert ecosystems dust combined [18,108]. The use of dry cooling, which reduces water deposition rates are sufficiently high as to adversely impact solar consumption by 90% to 95%, is a viable option in water-limited power generation [67,98]. ecosystems. Historically, reduced efficiency and higher startup Challenges to manage dust loads may be amplified by increases costs have been an economic deterrent to dry cooling [108]. in dust production related to land-use change, climate change (e. However, Holbert and Haverkamp [53] found that dry cooling g., increases in aridity) or disturbance to biological soil crusts (e.g., startup costs are offset by 87–227% over a 20-year time interval, fires, grazing, agriculture, energy exploration/development; [13]; owing to cost savings in water use and consumption. Global Field et al.[37];[95]). Even if USSE-related dust production is kept regions already water stressed, such as many arid and semiarid at bay, climate models predict an increase in aridity and recurrent habitats, may be vulnerable to changes in local hydrology [133], droughts in dryland regions of the world (e.g., [109]), which may such as those incurred by USSE activities. In water-constrained enhance soil erosion by wind and subsequent dust emissions. As areas, the deployment of USSE projects may also conflict with the these emissions can compromise the success of a USSE installation use of water by other human activities (e.g., domestic use, itself when they reduce its potential to generate electricity, agriculture), at least at the local scale [18,108]. Ultimately, the effective dust management is advantageous to ensure efficient choice of dry or wet cooling in a CSP plant can lead to highly power generation while minimizing deleterious environmental divergent hydrological impacts for USSE facilities. and health impacts.

2.3. Soil erosion, aeolian sediment transport, and feedbacks to 2.4. Human health and air quality energetic efficiency As with the development of any large-scale industrial facility, Aridlands, where USSE facilities are often concentrated [51],are the construction of USSE power plants can pose hazards to air also areas where high winds result in aeolian transport of sand quality, the health of plant employees, and the public [122]. Such and dust. Some of that sediment transport is controlled by desert hazards include the release of soil-borne pathogens [91], increases R.R. Hernandez et al. / Renewable and Sustainable Energy Reviews 29 (2014) 766–779 771

in air particulate matter (including PM2.5, [46,100]), decreases in also experience greater edge effects. Sites at different stages of visibility for drivers on nearby roads, and the contamination of vegetative recovery have exhibited distinct recolonization pat- water reservoirs [70]. For example, disturbance of soils in drylands terns, with lower native and higher introduced species diversity at of North and South America, which are places targeted for USSE, primary successional stages and an increase in native diversity at aids transmission of Coccidioides immitis, a fungus causing Valley mid- and late-successional stages [20]. The ecological effects of Fever in humans [10]. In areas where surface soil contains traces of transmission lines and corridors have proven to be varied and chemical and radioactive contaminants (e.g., radionucleotides, depend on a multitude factors, making appropriate siting crucial. agrochemical residues), increased aeolian transport resulting from soil disturbances increases contaminant concentrations in air- 2.6. Land-use and land-cover change borne dust [95]. During the decommissioning phase, PV cells can be recycled to 2.6.1. Land-use dynamics of energy systems prevent environmental contamination due to toxic materials Land and energy are inextricably linked [25]. When energy contained within the cell, including cadmium, arsenic, and silica systems are developed, biophysical characteristics of the land dust [144,145]. In the case of inappropriate handling or damaged may change (land-cover change, m2), the human use or intent cells, these industrial wastes can become exposed, which can be applied to the land may change (land-use change, m2), and the hazardous to the public and environment [144]. For example, land may be used for a specific duration of time (land occupa- inhalation of silica dust over long periods of time can lead to tion, m2 xyr; [40,64]). Terrestrial ecosystems vary in their net silicosis, a disease that causes scar tissue in the lungs and primary productivity (rate of accumulation of organic C in respiratory decline. In severe cases, it can be fatal [148].In plants), from tropical evergreen forests (1 to 3.2 kg/m2/yr1)to addition, chemical spills of materials such as dust suppressants, deserts (up to 0.6 kg/m2/yr1), and in their ability to sequester C coolant liquids, heat transfer fluids, and herbicides can pollute in soil [105]. When land-use and land-cover change occurs – for surface ground water and deep water reservoirs [70,126]. example, when vegetation or biological soil crust is cleared On rooftops, solar PV panels have also been shown to reduce roof or when soils are disturbed – above- and below-ground pools heat flux, conferring energy savings and increases in human comfort may release C back into the atmosphere as carbon dioxide from cooling [31]. In that vein, the insulating properties of rooftop (CO ; [26]). Hence, developing energy-related infrastructure solar PV may serve co-beneficially to mitigate heat wave-related 2 on previously disturbed or contaminated land may result in illness and mortality [131].Thefire hazard potential of both rooftop lower net C losses than infrastructure erected on undisturbed and ground-mounted USSE infrastructural materials (e.g., phosphine, lands [26,62,89]. diborane, cadmium), and their proper disposal, presents an additional Other key land-use characteristics of energy include land-use challenge to minimizing the environmental impacts of USSE facilities efficiency and reversibility. Land-use efficiency (e.g., watts per [43]. This is particularly true in light of the dramatic increases in the square meter, /m2)defines the installation′s power relative to its frequency and intensity of wildland fires in arid and semiarid regions footprint; the “footprint” being the land area transformed or of the world as a result of climate change ([134],[15]). impacted by the installation throughout the energy system′s complete conversion chain [40,51]. As energy systems may impact 2.5. Ecological impacts of transmission lines and corridors land through materials exploration, materials extraction and acquisition, processing, manufacture, construction, production, Centralized USSE operations require transmission of generated operation and maintenance, refinement, distribution, decommis- electricity to population centers where consumption occurs. This sioning, and disposal, energy footprints can become incrementally necessitates the development of expanded transmission infrastruc- high [40]. Some of this land may be utilized for energy in such a ture, the availability of which has not kept up with demand [21,30]. way that returning to a pre-disturbed state necessitates energy As of 2007, over 333 kilometers (km; 207,000 miles) of high-voltage input or time, or both, whereas other uses are so dramatic that transmission lines (4230 kV) were constructed in the US electricity incurred changes are irreversible [79]. Irreversibility cost assess- transmission system [78] and this number is expected to rise as ments can be employed to monetize restoration and irreversibil- transmission infrastructure expands to growing population centers ity; a function of the original land cover type and properties of the and connects with new renewable energy sources. As the potential land-use and land-cover change incurred [138,141]. for solar resources in other countries are being discovered so too are the plans to harness that energy and transmit it across international borders [27]; such plans are being actively developed to transmit 2.6.2. Land-use of utility-scale solar energy energy from Middle Eastern and North African regions to European Likely due to its nascent expansion [9], studies evaluating land- countries (requiring over 78,000 km of transmission lines by project use characteristics of USSE systems are relatively recent, few, and completion in 2050; [124]). Although essential for transporting focused geographically. Hsu et al. [54] described the complete energy energy, the construction of such extensive transmission line net- conversion chain of PV USSE systems, which necessitates materials works has both long- and short-term ecological effects, including acquisition, infrastructure and module manufacture, construction, displacement of wildlife, removal of vegetative cover, and degrada- operation and maintenance, material disposal, and decommissioning. tion of habitat quality [8], the degree of which may depend on land- The complete energy conversion chain of CSP is similar, but compli- use history, topography, and physical features of the sites, as well as cated by auxiliary natural gas and electricity consumption [16]. productivity and vegetation types. For example, Lathrop and Arch- Fthenakis and Kim [40] stated that indirect land impacts related to bold [66] estimated that biomass recovery at Mojave Desert sites materials (e.g., modules and balance-of-system) and energy for PV is disturbed for transmission line tower construction might take 100 negligible – between 22.5 and 25.9 m2/GWh1 – compared to direct years whereas recovery of disturbed transects directly beneath the land use. Data on land occupation are rare; however, the lifetime of transmission lines might take 20 years. USSE infrastructure, including modules, is typically assumed to be Fragmentation created by transmission corridors in forested between 30 and 60 years [40]. habitats may displace permanent resident species and disrupt Studies targeting the direct impact of USSE on land-cover regular dispersal patterns [7,97,107]. While wide transmission change are few [51,143,149]. Furthermore, factors controlling corridors may facilitate new habitat types resulting in higher sequestration of C in soils, particularly in aridlands, are not well diversity or the introduction of new communities [7,58,81], they understood [72,106], complicating the ability to quantify C losses 772 R.R. Hernandez et al. / Renewable and Sustainable Energy Reviews 29 (2014) 766–779 from USSE-related land-cover changes in the ecosystems where Management land has pending applications for USSE develop- they are most likely to occur [51]. In western US, 97,000 ha (ha) of ment. If constructed, creosote-white bursage desert scrub, the federal lands were approved or have pending leases for the Mojave mid-elevation mixed desert scrub, and over 10,000 ha of development of USSE while over 18 million ha of land in this desert tortoise habitat would be converted (Cameron et al., 2012). region were identified as suitable for USSE development [135].In Land-use efficiency of USSE is determined by the architec- the same region, Pocewicz et al. [92] found that USSE development tural and infrastructural design and capacity of the power plant may impact shrublands greater than any other ecosystem type, but indirectly influenced by a project′s geography, capacity with estimates of conversion ranging from 0.60 to 19.9 million ha, factor, technology type, and developer priorities. Hernandez et and especially for North American shrubland ecosystems. Smaller al. [51] found the nominal LUE efficiency of USSE in California to leases on grasslands and wetland ecosystems were approved, and be 35 W/m2 where a capacity factor of 13% and 33% would therefore may also be impacted but to a lesser extent. Hernandez generate a realized LUE of approximately 4.6 and 11.2 W m2 for et al. [51] found that USSE (420 MW; planned, under construc- PV and CSP, respectively. Fthenakis and Kim [40] used a nominal tion, and operating) in California may impact approximately packing factor (based on a single footprint specification) to 86,000 ha; concentrated in the agricultural center of the state determine the land use efficiency of PV and their results, (the Central Valley) and the arid, interior of southern California. ranging between 229 and 552 m2/GWh1,werecomparable In the Mojave Desert, over 220,000 ha of Bureau of Land to [51].

Fig. 4. Impact of temperature on global photovoltaic solar energy potential. In general, photovoltaic (PV) solar energy output increases with increasing irradiance but decreases with increasing ambient temperatures. These maps show (a) the global potential of PV energy (kWh/kW PV) for a crystalline silicon (c-Si) module, the most widely employed in the current market, without considering temperature effect, and (b) the global potential of PV energy (kWh/kW pV) for a crystalline silicon (c-Si) module including temperature effect. High irradiance coupled with low temperatures render the Himalayas, the Southern Andes, and Antarctica high in potential, 41800 kWh/kW. High temperatures reduce PV solar energy potential in places including southwest United States deserts, northern Africa, and northern Australia. Both (a) and (b) include impacts from cloud cover (maps reprinted from Kawajiri et al. [59]). Not well understood is how changes in land surface temperatures from climate change, especially heat waves, will impact future global PV energy output. R.R. Hernandez et al. / Renewable and Sustainable Energy Reviews 29 (2014) 766–779 773

To date, no study has evaluated how USSE land use efficiency potential, as PV technologies increase in efficiency over time so too (W/m2) and layout – the infrastructural and architectural design of will their effective albedo. a USSE power plant – may impact ecosystem recovery or reversi- bility. However, the natural recovery of aridlands and other ecosystems after disturbance can be exceptionally slow. For 3.2. Utility-scale solar energy and surface roughness example, leases for USSE development on public land in southern California deserts are typically at the decadal-scale, while com- Changes in radiative balance can also occur due to changes in plete ecosystem recovery from USSE activities there may require surface roughness. In the built environment, changes in roughness over 3000 years [69]. length (mean horizontal wind speed near the ground) is likely to be negligible given that PV panels are typically roof-embedded or 2.6.3. Comparing land-use across all energy systems resting slightly above the roof. In natural environments, specifi- Land-use and land-cover change impacts from USSE are rela- cally deserts, roughness length typically increases given the tall tively small when compared to other energy systems [146].Infive infrastructure of USSE plants. Indeed, Millstein and Menon [76] ecosystems in western United States, Copeland et al. [21] found found that the solar arrays influenced local and regional wind that actively producing oil and gas leases impact 20.7 million ha of dynamics up to 300 km away. land (4.5% of each terrestrial ecosystem evaluated) but the total potential for lands to be disturbed exceeded 50 million ha (11.1%). In contrast, potential land-cover change impacts from USSE 3.3. Utility-scale solar energy and climate change development was o1% of all ecosystems combined. In terms of land-use efficiency, PV energy systems generate the greatest Complicating our understanding of land-atmosphere interac- amount of power per area among renewables, including wind, tions with USSE is climate change. Arguably one of the biggest hydroelectric, and biomass [40,51]. Notably, ground-mounted PV challenges to the deployment of these facilities will be anticipating installations have a higher land use efficiency (when incorporating reductions in water resources in areas that are already water- both direct and indirect effects [e.g., resource extraction]) than stressed [80]. In 2009, all operating CSP facilities in the US were surface coal mining, which is how 70% of all coal in the United wet cooled [18]. Reductions in water availability will have con- States is extracted [40]. These results underscore the environ- sequences for both USSE facility operation and dust deposition on mental potential solar energy development may have on land- mirrors or panels (utility-scale and distributed). In places where cover and land-use change impacts, relative to carbon-intensive more frequent, intense storms may occur, managing operational energy and other renewable energy sources. and ecological impacts of erosion will be an exigent concern [93]. Another part of the challenge lies in the shifting of climate envelopes and incidence of extreme weather. Photovoltaic tech- 3. Utility-scale solar energy, land-atmosphere interactions, nologies use both direct and diffuse light to convert energy from and climate change the sun into electricity, but high ambient temperatures reduce panel efficiency almost linearly (Fig. 4). Consequently, cool places Assessments of USSE impacts on land-atmosphere interactions, with high irradiance are the best locations for capturing solar with especially those with climate feedbacks, are increasing in number. PV [59]. Currently, combined uncertainty (i.e., standard deviation) While there are two principal types of solar technologies (i.e., PV of PV yield is roughly 8% during the PV system lifetime [123]. and CSP) recent research on land-atmosphere attributes of USSE Uncertainty may increase if climate change projections are taken have focused largely on PV [31,76,121], given their relatively larger into consideration. Concentrating solar power efficiency increases deployment globally (65 GW of PV versus 1.5 GW of CSP; Interna- linearly with increasing ambient temperature and proportionally tional Energy Agency, 2013). to direct light and therefore changes in climate also impact CSP output. Indeed, site-specific favorability for PV and CSP are 3.1. Utility-scale solar energy and albedo projected to vary over time under different climate change scenarios; for example, CSP may increase up to 10% in Europe The radiative balance at the land-atmosphere interface can under the Intergovernmental Panel on Climate Change A1B sce- shift when the albedo of a PV solar installation differs from the nario [22]. former background albedo. Given their absorptivity, PV panels The substitution of carbon-intensive energy sources for solar have an effective albedo (averaging 0.18–0.23), a function of its energy has enormous potential to mitigate climate change by directly inherent reflectivity and solar conversion efficiency [83]. Using a reducing greenhouse gas emissions [150].IntheUS,Zhaietal.[137] fully coupled regional climate model, Millstein and Menon [76] modeled a reduction of CO2 emissions from 6.5% and up to 18.8%, if showed that a 1 TW PV USSE installation (at 11% efficiency) in the PV were to comprise 10% of the grid. Recently, a suite of studies Mojave Desert would decrease desert surface albedo, thereby harmonized (i.e., standardized and performed a meta-analysis of data increasing temperatures up to 0.4 1C. In cities, albedos average from a large number of studies) current life cycle analysis literature 0.15 to 0.22 and consequently installed PV arrays can potentially to evaluate life cycle greenhouse gas emissions from various solar increase albedo for a cooling effect. Taha [121] modeled a high- energy technologies, including upstream (e.g., resource and raw density deployment of roof-mounted PV panels (i.e., a distributed material acquisition, product manufacturing), operational, and down- scheme) in the Los Angeles Basin and found no adverse impacts on stream (e.g., selling and distribution of product, decommissioning air temperature or the urban heat island and predicted up to 0.2 1C and disposal) processes (Table 1). Photovoltaic solar technologies 1 decrease in air temperatures under higher efficiency panels. ranged from 14 to 45 g CO2-eq kWh [54,60],whereCO2-eq is the Although local- and regional-scale land-atmosphere impacts carbon dioxide equivalent, a measure for quantifying the climate- are important to consider, particularly in environmentally sensi- forcing strength of greenhouse gases by normalizing for the amount tive ecosystems, the global-scale substitution of carbon-intensive equivalent to CO2. Concentrating solar power ranged from 26 to 38 g 1 energy for solar energy cannot be understated. Nemet [84] found CO2-eq kWh , for parabolic trough and power tower, respectively that when PV is substituted for fossil fuels at the global scale, the [16]. These emission values were a magnitude of order less reduced radiative forcing is 30 times larger than the increase in than greenhouse gas emissions from coal, gas, or oil Varun and radiative forcing from reduced albedo. Further underscoring their Prakash [132]. 774 R.R. Hernandez et al. / Renewable and Sustainable Energy Reviews 29 (2014) 766–779

4. Utility-scale solar energy co-benefit opportunities architecture and design that serves to concomitantly conserve water and land resources (Fig. 5). Solar energy is one of the most promising alternatives to fossil fuels, especially as an attractive climate change mitigation option [150]. Clear-cut advantages of solar energy such as utilizing the sun as a renewable source of electrons and heat, and the reduction 4.1. Utilization of degraded lands of air and water pollution by fossil fuels, can be complemented by additional environmental co-benefit opportunities [118,127]. Degraded lands comprise approximately one-fourth of all land Opportunities include, but are not limited to the (1) utilization of on Earth [63]. The development of “brightfields” on degraded degraded lands, (2) co-location of solar panels with agriculture, lands [153]—including brownfields, landfills, mine sites, and other (3) hybrid power systems, (4) floatovoltaics, and (5) novel panel types of contaminated lands—confer several environmental co- benefits, including obviating additional land-use or land-cover change. For example, 12,000 ha of salt-contaminated agricultural Table 1 Comparison of life cycle emissions for solar (grams of carbon dioxide equivalent per land in the San Joaquin Valley (California, USA; Fig. 5a) are planned kWh) and conventional, carbon-intensive (grams of carbon dioxide per kWh) for conversion into a 2.4 GW solar power plant (www.westlands energy generation. solarpark.com). Employing water-efficient PV solar technology, the park′s location stands to divert large amounts of water to active, Conventional systems Renewable systemsa water-stressed agricultural sites nearby; hence garnering broad

System g-CO2/kWh System g-CO2-eq/kWh support from various interest groups. Utilizing degraded land can offer additional environmental c b Coal 975 Concentrating solar power benefits when reclamation of these lands is prioritized. On-site Gasc 608 Parabolic troughd 26 landscaping using native plants and soil amendments can add to Oilc 742 Power towerd 38 Nuclearc 24 Photovoltaics ecosystem service provisioning (e.g., soil stability, C sequestration) Crystalline-silicone 45 without the use of additional water and fertilizer inputs. Thin-film amorphous siliconf 21 A 550 MW PV power plant spread over 1400 ha of private, non- fi f Thin- lm cadmium telluride 14 prime agricultural land in San Luis Obispo (California, USA) will Copper indium gallium 27 fi fi Diselenidef use economical, thin- lm PV cells that operate ef ciently in the relatively low light conditions characterizing this area (Fig. 5b). a Median values, assuming life span of 30 years. This mesic site reduces water consumption for panel cleaning and b Excludes auxiliary natural gas combustion and electricity consumption. is also the location of an effort to re-establish the native grasslands c Varun and Prakash [132]. that once dominated [6]. Under and around the panels, sheep will d [16]. e [55]. graze the taller grasses every two months to prevent obstruction f [61]. of panels.

Fig. 5. Environmental co-benefit opportunities of utility-scale photovoltaic solar energy: ((a) and (b)) Utilization of degraded lands, (c) Co-locating solar energy and agriculture, and (d) Photo credits: Westlands Solar Park, Optisolar, Bert Bostelmann/Getty Images, [111]. R.R. Hernandez et al. / Renewable and Sustainable Energy Reviews 29 (2014) 766–779 775

4.2. Co-location with agriculture benefits were included in the cost benefit analysis. Photovoltaic noise barriers originated in Switzerland in 1989, and today over Environmental co-benefits can occur when existing agricultural 9 MW of PV noise barriers have been erected alongside rail and land is co-located with solar. With potential minimal risks to food highway systems in Europe, Australia, and China. security, co-location schemes can reduce land deficits for energy, In addition to ground-mounted panels, PV installation on food, and fiber production [25]. A preliminary study by Dahlin rooftops has enhanced solar energy production as well [118]. et al. [24] found that US electricity production could be met by Government incentives known as feed-in-tariffs used in 48 coun- utilizing approximately 11% of of US cropped land. The co- tries encourage the use and growth of renewable energy in both existence of grazing habitat for livestock, such as sheep and goats, commercial and residential sectors, including PV deployment on may curtail the need for vegetation removal and maintenance, or rooftops as it has the potential to contribute energy on a utility both, and limit erosion, while supporting both energy and food/ scale. For example, the Canadian province of Ontario has begun a fiber production (Fig. 5c). Yet such sites need not be agricultural large-scale PV integration into infrastructure since 2009 and it is land sensu stricto. For example, Japan announced a co-location estimated that its total area of viable rooftops can produce up to plan to diversify their grid by integrating 30 MW of PV in the 30 GW of solar energy as compared to 90 GW from ground- unoccupied spaces adjacent to and on top of livestock barns, mounted panels in utility-scale solar plants [118]. Similar to agricultural distribution centers, and parking lots [84]. Where land Ontario, USSE companies in Amsterdam are capitalizing on PV for agriculture is limited in aridlands, coupled USSE infrastructure integration into the built-environment through rooftop installa- and biofuel cultivation has been suggested as a strategy to tions on residential homes [155]. minimize the socioeconomic and environmental issues resulting While land and rooftop-based PV installations are typically from biofuel cultivation in agricultural lands [96]. connected to a grid system, PV panels can also be used to generate power for off-grid domestic and non-domestic environments 4.3. Hybrid power systems [156]. This setup offers a reliable source of energy for communities and villages in remote locations that lack access to a central utility The United States Department of Energy [130] estimates that power-line. Off-grid PV systems are vital to rural communities by more than one million ha of land would be required in the US to providing electricity for basic needs and have a particularly large achieve the USSE 2030 SunShot scenario of 642 TW h. In the US impact in developing countries such as India, Indonesia, Sri Lanka, and other countries where land is limited, co-location with other and Kenya, where only a small percent of rural communities are energy systems (e.g., wind, biomass, conventional thermal or grid-connected [147,154]. natural gas power plants) may prove advantageous [115,120]. Hybridization and optimization methodologies for co-locating solar and wind power are currently being implemented in diverse 5. Minimizing adverse impacts of solar energy: Permitting and geographic regions [115,120]: Charanka village in India provides an regulatory implications example of a wind-solar colocation region with 0.5 GW of com- bined wind and solar energy capacity [113]; a conventional fossil Permitting and regulatory constraints for USSE vary with land fuel 44 MW coal plant in Cameo, Colorado has been co-located ownership (e.g., public versus private land), ecological character- with a 4 MW USSE trough for preheating feed water (IEA, [56]); istics (e.g., undisturbed versus previously degraded, critical habitat and, Ordos City, Mongolia is co-locating the largest USSE facility for rare species) and cultural significance [152]. From the perspec- in the world at a capacity of 2 GW PV alongside nearby wind and tive of the public, the benefits of renewable energy development coal facilities [28]. Uncovering novel synergies between solar and ought to be weighed against the loss of ecological function, loss other energy sources will continue to require diverse project imple- of public access, and the loss of irreplaceable cultural resources mentations and industry-relevant field experiments, along with [126,151]. From a perspective of energy development alone, modeling studies on the energetic advantages and trade-offs of possible delays from permitting requirements and regulatory co-locating USSE with other facilities. reviews may be seen as having negative effects on financial returns. 4.4. Floatovoltaics Like other forms of renewable energy, each USSE project will ineluctably have its own unique set of social, cultural, environ- A unique water-based design element is the use of “floatovol- mental, technical, and political characteristics [152]. Project imple- taics”. Innovative designs for reservoir-based PV modules – such as mentation may be further complicated by wavering market prices polyethylene floating arrays that utilize elastic fasteners to adapt for land acquisition and materials in addition to environmental to varying water levels – are beginning to proliferate globally [36]. regulations and legislation that may vary across county, state, and Such water-borne PV systems are also being deployed in diverse national boundaries. Collectively, the wide variation in require- water features including the muddy waters of a wastewater ments to develop USSE marks a discrepancy in solar energy treatment site (Richmond, CA; NRG [86]), a pond where electricity implementation amongst different regions. is generated for the adjacent vineyard located in the Napa Valley, In general, policies underlying the development of energy systems California [116,117], and an irrigation canal in Gujarat, India in all countries have yet to address all key impacts and externalities. (Fig. 5d; [112]). This 750-m stretch of irrigation canal in India Consequently, all the actors and entities involved in a single enterprise has been covered by 1 MW of PV panels, thereby reducing the may be working independently to minimize adverse impacts in ways need for land transformation and conserving roughly 9-million not regulated or incentivized by policy. Ways to minimize impacts liters of water per year owing to reduced evaporation. include: (1) understanding the environmental implications of siting decisions using adequate inventories of species and processes Tsoutsos 4.5. Photovoltaics in design and architecture et al. [126], (2) monetizing the actual value of natural capital and ecosystem services attributed to a parcel of land, (3) siting USSE Integrating PVs into infrastructure and architectural elements systems on land that maximizes energetic output and minimizes can create numerous co-benefits, first by obviating the need for economic and environmental costs Tsoutsos et al. [126] [19]), (4) hav- additional land-use or land-cover change. One study [103] found ing individuals and entities involved with long-term commitments PV noise barriers to be economically profitable when ecological to the project, and (5) requiring developers to internalize costs. 776 R.R. 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In addition, standardizing the rigidity and quality of regulations for all 6.3. Research questions addressing utility-scale solar energy USSE projects may serve to streamline USSE development. co-benefit opportunities

Utilization of degraded lands 6. Solar energy and the environment: Future research To what extent are USSE power plants erected on degraded lands? Below, we suggest a list of research questions to springboard Does USSE infrastructure (e.g., shading) and maintenance future studies aimed at expanding our understanding of the requirements (e.g., panel washing) increase soil C sequestration interaction between USSE and the environment. We have devel- in degraded lands? oped these questions to bridge empirical gaps that were identified Co-location with agriculture as a result of this review. Where applicable, we have provided What are the environmental tradeoffs between allocating lands citations for studies that have addressed each question, in part, or to USSE development versus agriculture? existing studies that prompted our proposed research questions. What are the socioeconomic consequences of USSE develop- Gaps in the literature where empirical research is lacking are ment in agricultural areas? How does USSE development indicated by the absence of citations. impact local food security and employment opportunities? Can transpiration from vegetation/agriculture reduce solar panel temperature thereby increasing efficiency? 6.1. Research questions addressing environmental impacts When combining USSE systems and agriculture, what are the of utility-scale solar energy systems effects on crop yield? [24] Hybrid power systems Direct, indirect, and regional effects on biodiversity What environmental and economic advantages and disadvan- How do infrastructural design, module configuration, and tages lie in the co-location of solar energy with other energy shape of a USSE power plant affect biodiversity? technologies? To what degree are native species impacted by USSE power How can solar hybrid energy systems be optimized? [115,120] plants? ([75]; Lovich and Ennen, [70]) Are there certain taxa, Photovoltaics in design and architecture life histories, or functional types that are more compatible with What is the technical potential of USSE as deployed in the built USSE than others? environment? To what degree does USSE infrastructure serve as a corridor or What is the cost-benefit of roof-embedded and roof-top solar, impasse for the movement of species and their genes? including savings derived from reduced cooling needs? [31] Water use and consumption What are the economic and environmental impacts of distrib- How much water is displaced from agricultural and domestic uted/built environment solar schemes versus USSE in undeve- use for USSE construction and operation? [44] loped lands? Is there an ideal portfolio ratio? Soil erosion, aeolian sediment transport, and feedbacks to energetic efficiency What is the relationship among USSE electrical generation, location, and dust? Does vegetation beneath panels reduce dust deposition on 6.4. Research questions addressing permitting and regulatory modules? implications Human health and air quality What are best practices for use of dust suppressants, coolant How do environmental regulations and legislation impacting liquids, heat transfer fluids, and herbicides at USSE facilities? USSE development vary across county, state, and national (Lovich and Ennen,)[70]. boundaries? Ecological impacts of transmission lines and corridors How effective are renewable energy policy measures in facil- How can existing transmission infrastructure and corridors be itating USSE growth? [118] maximized for USSE development? [39] Land-use and land-cover change 7. Conclusion What are the land-use and land-cover impacts of USSE globally and compared to other energy systems? [40,51,92] Utility-scale solar energy systems are on the rise worldwide, an What is the relationship between land use efficiency and expansion fueled by technological advances, policy changes, and reversibility? For example, is it better to arrange modules as the urgent need to reduce both our dependence on carbon- close together as possible or spread them out? [51] intensive sources of energy and the emission of greenhouse gases to the atmosphere. Recently, a growing interest among scientists, solar energy developers, land managers, and policy makers to 6.2. Research questions addressing utility-scale solar energy, understand the environmental impacts – both beneficial and land-atmosphere interactions, and climate change adverse – of USSE, from local to global scales, has engendered novel research and findings. This review synthesizes this body of Utility-scale solar energy and albedo knowledge, which conceptually spans numerous disciplines and To what extent can the spatial arrangement and materials of crosses multiple interdisciplinary boundaries. USSE infrastructure be used to enhance cooling (e.g., in urban The disadvantageous environmental impacts of USSE have not heat islands)? ([31]; Taha, In press) heretofore been carefully evaluated nor weighted against the numer- Utility-scale solar energy and surface roughness ous environmental benefits – particularly in mitigating climate How does USSE impact local and regional wind dynamics [76] change – and co-benefits that solar energy systems offer. Indeed, Utility-scale solar energy and climate change several characteristics and development strategies of USSE systems How will climate change impact utility-scale solar energy? [22] have low environmental impacts relative to other energy systems, What is the potential of USSE to mitigate climate change in including other renewable energy technologies. Major challenges to various regions worldwide and globally [137] the widespread deployment of USSE installations remain in R.R. Hernandez et al. / Renewable and Sustainable Energy Reviews 29 (2014) 766–779 777 technology, research, and policy. Overcoming such challenges, high- [19] CBI (Conservation Biology Institute), 2010. Recommendations of independent lighted in the previous sections, will require multidisciplinary science advisors for the California Desert Renewable Energy Conservation Plan (DRECP). CBI. Available at 〈http://www.energy.ca.gov/2010publications/DRECP- approaches, perspectives, and collaborations. This review serves to 1000-2010-008/DRECP-1000-2010-008-F.PDF〉 (accessed 06.30.13). induce communication across relatively disparate disciplines but [20] Clarke DJ, White JG. 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